1. Field of the Invention
The present invention relates to a self-extinguishing cable and to a fire retardant composition used therein.
Particularly, the invention describes a self-extinguishing cable comprising at least a conductor and at least a conductor-coating layer comprising a fire retardant composition suitable to provide the cable with self-extinguishing properties.
2. Description of the Related Art
The improvement of the characteristics of flame retardant materials is continuously demanded by skilled persons in numerous fields. In particular, there have been prepared materials which, added to organic substances, specifically to per se combustible plastic materials, are suitable to prevent or at least to delay the combustion events, especially the flame propagation when these organic substances are subject either to such high temperatures as to cause their combustion or to the direct action of the flame.
A first solution proposed with the aim of reducing the flame propagation is to add the plastic material with halogenated organic compounds, particularly chlorinated and/or brominated organic compounds, possibly in admixture with antimony oxides, as flame retardant agents. It is believed that such halogenated compounds carry out their action by affecting the radical mechanisms occurring in the gaseous phase during the combustion. Although the resulting compositions show good flame retardant properties, the halogenated flame retardant agents exhibit a number of drawbacks since they partially decompose during the processing of the polymer composition, producing halogenated gases which are toxic for workers and corrosive for the metal parts of the machine tools. Moreover, when they are subject to the direct action of the flame, their combustion produces huge quantities of fumes containing toxic gases. Similar inconvenients are noticed when polyvinylchloride (PVC) added with antimony trioxide as a polymer base is used.
Inorganic charges free from halogens having flame retardant properties, such as metal hydrated oxides or hydroxides, particularly aluminium or magnesium hydroxide, are also known. At high temperatures, these products undergo an endothermic decomposition process which generates water, thus depriving the substrate from the heat and, therefore, slowing the pyrolysis reactions of the plastic material. Nevertheless, such inorganic agents fulfil their task only if they are present in remarkable amounts (generally at least 70-120 parts by weight calculated on 100 parts by weight of the base polymer), though to the detriment of the mechanical properties of the flame retardant composition. Particularly, the values of the elongation and stress at break result sensibly lowered in comparison with the polymer material per se.
Further, other fire retardant systems free from halogens, commonly known as xe2x80x9cintumescent systemsxe2x80x9d, have been developed, which, added to a plastic material, are suitable to cause, when combustion occurs, the expansion of the material itself and, at the same time, the formation of an expanded charcoal layer (xe2x80x9ccharxe2x80x9d) which prevents oxygen from passing from the outside to the inside, thereby blocking the combustion process for lack of comburent. Besides, the expansion of the plastic material, causing a volume increase, is believed to reduce the emission of flammable gases deriving from the decomposition of the organic substances and, at the same time, to protect the plastic material from the strong thermal irradiation coming from the zone of combustion.
Intumescent systems generally consist of a foaming agent and a dehydrating agent, optionally in the presence of a carbonization agent.
Usually, the foaming agent is a nitrogen compound, for instance urea or melamine, which develops non-flammable gases (particularly nitrogen) and contributes to the formation of char.
The dehydrating agent takes the water away from the substrate under combustion, making the carbonisation thereof easier, and generally consists of phosphorated compounds, for instance ammonium polyphosphates or melamine sulphates or phosphates.
The optionally present carbonization agent substantially contributes to the char formation and usually consists of polyhydroxylated organic substances such as sugars.
Patent application EP-A-0629677 discloses flame retardant compositions free from halogens comprising, in addition to an intumescent system having the characteristics already described, an aluminosilicate, particularly a zeolite.
Also patent application WO 95/16736 discloses compositions comprising a flame retardant substantially free from halogens and wherein the polymer base component consists of styrene polymers substantially free from oxygen. The flame retardant comprises at least one of the following additives: red phosphorus, ammonium polyphosphate, melamine phosphate, pyrophosphate or cyanurate, or resorcinol bis(diphenylphosphate), magnesium hydroxide and a thermoplastic elastomer.
U.S. Pat. No. 5,698,323 describes a further intumescent system free from halogens and antimony, used for making energy or telecommunication cables, which comprises magnesium hydroxide or alumina trihydrate, zinc oxide and red phosphorus; the polymer base component consists of ethylene copolymers with acryl or vinyl esters.
On the basis of the Applicant""s experience, the known fire retardant systems show a decreased self-extinguishing ability, when the combustion is repeated. In other words, the efficacy of fire retardant systems tends to be exhausted after the temperature increase caused by the first application of the flame. In fact, such temperature increase can reduce the sample""s abilities of self-extinguishing to such an extent as to make the sample combustible again, with obvious undesirable drawbacks.
The technical problem of obtaining a cable endowed with good self-extinguishing properties, also after repeated combustions, without impairing the mechanical properties of the cable, remains therefore still unsolved.
The Applicant has unexpectedly found that the aforesaid technical problem can be solved by using, for at least one of the conductor-coating layer of the cable, a flame retardant composition comprising a polymer base material in admixture with melamine or a derivative thereof, red phosphorus, magnesium and/or aluminium hydroxyde and a zeolite, in predetermined amounts.
Particularly, the Applicant has found that the self-extinguishing ability of a cable comprising such a fire retardant composition, also upon repeated combustions, turns out to be improved without impairing the mechanical properties of the cable.
The practical importance of the resistance of a cable endowed with such characteristics, i.e. able to resist to subsequent combustions, is apparent in real fire conditions.
In fact, the cable is generally located in an environment wherein it is in contact with other combustible objects, such as plastic raceways, panels, cables of different nature, as well as other objects having self-extinguishing characteristics which can be different from the ones of the cable.
It can therefore happen that the cable endowed with self-extinguishing properties becomes self-extinguished after the first ignition and that, however, being in contact with other objects which are still under combustion, it is found in such conditions that it must be re-ignited. This can be repeated more times and it is therefore apparent that a peculiar system able to self-extinguish after subsequent ignitions has a remarkable practical importance.
According to a first aspect, therefore, the invention concerns a self-extinguishing cable comprising at least one conductor and at least a coating layer comprising a fire retardant composition comprising:
(a) a polymer base material;
(b) melamine or a derivative thereof, in amounts equal to 5-50 parts by weight;
(c) red phosphorus, in amounts equal to 5-50 parts by weight;
(d) magnesium and/or aluminium hydroxide or hydrated oxide, in amounts equal to 10-150 parts by weight;
(e) a zeolite in amounts equal to 1-5 parts by weight;
said amounts being referred to 100 parts by weight of component (a).
Preferably, the polymer base material (a) can be selected from: polyolefins, copolymers of different olefins, copolymers of olefins with esters having ethylene insaturation, polyesters, polyethers, copolymers polyether/polyester, and mixtures thereof.
Examples of such polymers are: high density polyethylene (HDPE) (d=0.940-0.970 g/cm3), medium density polyethylene (MDPE) (d=0.926-0.940 g/cm3), low density polyethylene (LDPE) (d=0.910-0.926 g/cm3); copolymers of ethylene with alpha-olefins having from 3 to 12 carbon atoms (for instance 1-butene, 1-hexene, 1-octene and the like), particularly linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE) (d=0.860-0.910 g/cm3); polypropylene (PP); thermoplastic copolymers of propylene with another olefin, particularly ethylene; copolymers of ethylene with at least an ester selected from alkylacrylates, alkylmetacrylates and vinylcarboxylates, wherein the alkyl group, whether linear or branched, may have from 1 to 8, preferably from 1 to 4, carbon atoms, whereas the carboxyl group, whether linear or branched, may have from 2 to 8, preferably from 2 to 5, carbon atoms, particularly copolymers ethylene/vinylacetate (EVA), ethylene/ethylacrylate (EEA), ethylene/butylacrylate (EBA); elastomeric copolymers ethylene/alpha-olefins, particularly elastomeric copolymers ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM); and mixtures thereof.
Melamine derivatives suitable for implementing the invention can be selected from the inorganic and organic derivatives commonly used in the art such as, for instance, guanamines or melamine phosphates (phosphate, pyrophosphate, etc.), borate, sulphate, cyanurate.
In this description xe2x80x9czeolitexe2x80x9d is meant to indicate a natural or synthetic hydrated aluminosilicate having an open tridimensional crystal structure, wherein water molecules reversibly insert.
Particularly, zeolites can be represented by the following formula:
Mx/n[(AlO2)x,(SiO2)y].wH2O
wherein
M represents an alkali or earth-alkali metal cation, tetralkylammonium, preferably sodium or calcium;
x, y and w are integer numbers, the y/x ratio being a number greater than or equal to 1;
n is the cation valence, preferably 1 or 2.
According to a preferred aspect, the cable of the invention has amounts of components (b), (c), (d) and (e), always referred to 100 parts by weight of component. (a), of 10-30, 10-30, 20-100 and 1,5-3 parts by weight, respectively.
According to another aspect, the invention further concerns a fire retardant composition comprising:
(a) a polymer base material;
(b) melamine or a derivative thereof, in amounts equal to 5-50 parts by weight;
(c) red phosphorus, in amounts equal to 5-50 parts by weight;
(d) magnesium and/or aluminium hydroxide or hydrated oxide, in amounts equal to 10-150 parts by weight;
(e) a zeolite in amounts equal to 1-5 parts by weight;
said amounts being referred to 100 parts by weight of component (a).
The fire retardant composition according to the invention can be prepared by mixing the polymer components with additives according to techniques known in the art. The mixing can be carried out, for instance, by an internal mixer of the tangential (Banbury) or copenetrating rotor type, or by continuous mixers of the Ko-Kneader (Buss) type or of the co-rotating or counter-rotating double-screw type. The flame retardant compositions of the invention are preferably used in a non cross-linked form aiming at obtaining a coating having thermoplastic properties and therefore recyclable. It is also possible to carry out a total or partial cross-linking of the compositions of the invention according to known techniques, particularly by adding a radical initiator, for instance an organic peroxide.
The compositions of the invention can be used therefore to directly coat a conductor, or to make an external sheath on the conductor previously coated with at least an insulating layer. The coating step can be carried out for instance by extrusion. In case at least two layers are present, the extrusion can be carried out in several separate steps, for instance by extruding, in a first step, the internal layer on the conductor and, in a second step, the external layer on the internal one. Advantageously, the coating process can be made in one step, for instance by the xe2x80x9ctandemxe2x80x9d technique, wherein different single extruders, arranged in series, are used, or by co-extrusion with a single multiple extruding head.